Contemporary communications systems often employ an oscillator to create an output signal or to downconvert a received signal. Many conventional communications systems employ a voltage controlled oscillator (VCO) for this oscillator function.
In these circuits, the VCO is part of a phase-lock loop or other type of frequency or phase characteristic control loop. In the loop, the output of the VCO is conventionally controlled by a loop filter. The output of the loop filter is in turn controlled by an error signal which indicates a difference between an expected output and an eventual output of the system.
A VCO having a single segment has its output governed by static reactances within its structure. Thus, the frequency range of the single segment VCO is strictly limited to one and only one range of output, and one and only one range of input control voltages.
A conventional VCO without multiple segments outputs a signal having a particular frequency (or other frequency or phase characteristic) across a specified voltage input at a particular input. Accordingly, if a frequency is required that is outside the range of the output voltage of the loop filter or outside the range of the single output segment of the VCO, the particular output cannot be realized.
In some conventional communications system, the VCO can be made with dynamically controlled, configured, or switched reactances, thus allowing the VCO to operate at a different range of output given the same range of input voltages. In this manner, several ranges of output are possible for the VCO within the same range of voltages provided by the loop filter.
FIG. 1 is a graph depicting an output frequency of a multi-segment VCO graphed against input voltages. In this graph, a VCO can produce several segments, each having a different frequency range output based upon a single range of input voltage. In this manner, if the VCO is preset to operate on a segment 1, the input voltages over the range will produce the frequencies as depicted on that segment 1.
To achieve larger ranges of output from the VCO, the path with reactive elements may be altered, or the values of some of the reactive elements within the VCO may be switched. Accordingly, this would drive the output frequencies for the same range of voltages to another segment 2. In this manner, the limitations of the voltage range of the loop filter need not be a limitation on an output frequency. Further, the full range of frequencies as depicted in the segments 1 through 4 may be obtained using a single output voltage range of the associated loop filter of the system.
However, the ability of a communications system to freely operate amongst the segments is limited. In many conventional systems, the functional components used to perform selection of a particular segment are independent of the components used in the generation of an output signal. Further, the ability of the segment selection components to efficiently select a proper segment can also be inefficient in the manner in which the segment is eventually selected.
FIG. 2 is a schematic block diagram of a conventional communications system employing a variable segment VCO. A communications device 10 has a segment selection circuit 12, a VCO 14, and a loop filter 16. Upon initiation of operation, the segment selection circuit 12 inputs a digital word to the VCO 14. The particular digital word presented to the VCO 14 actuates or shunts switches in the VCO 14, thereby initializing the VCO 14 to operate at a particular output segment.
In many conventional systems, the output of the VCO 14 on a particular segment is then tested against the output that is wished. In one configuration, the segment selection circuit 12 selects a particular segment, and then inputs a voltage to the VCO 14 corresponding to the floor voltage of the voltage range associated with normal operation. Next, the segment selection circuit 12 inputs a ceiling voltage to the VCO 14 corresponding to the high voltage of the voltage range associated with normal operation. If the required operating point lies between the two outputs of the VCO 14 at these two points, as measured by the segment selection circuit 12, the segment selection circuit 12 can determine that the currently input digital word indicates the proper segment at which the VCO 14 should operate.
If the operating point is found not to reside between the outputs of the floor input voltage and the ceiling input voltage, the segment selection circuit 12 then increments or decrements the digital word input into the VCO 14. In this manner a new segment is selected and the test is run again using the newly chosen segment. In this manner the system 10 performs a somewhat inefficient search to find the proper segment on which the VCO 14 should operate.
In another manner of operation, the testing of the segment can be done using only the center point frequency of the selected segment. However, this does not eliminate the inefficiency associated with the linear search method to find the proper segment.
In some other conventional communications systems a binary search could be employed to determine the proper operating segment. In this case the segment selection circuit 12 would indicate a center segment to test from the variable segments as an input into the VCO 14. If it is found that the operation point of the VCO 14 is higher than the currently selected segment, the input word denoting the presently selected segment is made a ceiling. Next, the input to the VCO 14 is changed to indicate the center segment of the remaining segments. In this manner a binary search can be employed to search for the proper operation segment of the VCO 14.
In some communications standards, a communications device is expected to operate at more than one output frequency. Some of these communications standards allow a limited time in which to change the operating point of the output. Accordingly, an inefficient search for the proper operation segment and for the proper operating point on the segment for the VCO 14 may be pressured to meet the specifications of such multi-channel, dual mode, or multi-mode systems.
As newer communications standards evolve, the operational standards also typically call for more efficient use of the transmission time, as well as having multiple modes or operating channels. Thus, a VCO that is not efficiently finding a proper operational segment is also pressured through the reduction in timing consideration as well as explosion in possible multiple outputs described above.
Further, many conventional communications systems operate the segment selection circuit's interaction with the VCO 14 without regard to the other components of the conventional transmission loops. Thus, in these conventional communications systems, while the segment selection circuit 12 is deriving the proper segment at which the VCO 14 is to operate, the loop filter 16 will be disabled. Only when the proper operational segment has been found, does the loop filter 16 become operational. Thus the operations of the segment selection circuit will result in the decoupling of the normal operation path in the frequency or phase characteristic control loop. In this manner, the operational path of the transmission system 10 will be completely disabled while the search function is occurring.
One will note that the combination of the segment selection circuit and the VCO can form a closed loop during the selection process. This conventional usage forces the searching and testing to shut down the operational components of a phase-lock loop during the selection process, after which the selection circuit is disabled and shut down.